1. Field of the Invention
The invention relates to a headlight for automobiles, and more particular to a headlight with low beam and high beam radiating from the same poly-ellipsoid system (PES) for automobiles.
2. Description of the Related Art
In a conventional headlight for automobiles, due to the optical principles, normally, high beam and low beam do not radiate from the same PES. Referring to FIG. 1, to have low beam and high beam radiating from the same conventional headlight, a parabolic reflecting mirror 10 is in use. A low beam filament 14 and a high beam filament 15 are located on the optical axis 12 of the parabolic reflecting mirror 10. Since the optical paths for low beam and high beam are different, the low beam is reflected upward and a high beam radiates horizontally after being reflected by the parabolic reflecting mirror 10. The low beam thus designed forms a dazzling light for human being. In most of the European standards and regulations, an obvious boundary of brightness for low beam is necessary. Thus, this kind of headlight does not meet the requirement at all.
Another common headlight for automobiles adapts the design of locating the low beam filament and the high beam filament into two different PESs. An optical design for a high beam is shown in FIG. 2a and FIG. 2b. FIG. 2a is the vertical cross sectional view of the optical design for the high beam. On the optical axis 22 of a poly-ellipsoid reflecting mirror 20, a high beam filament 24 is located. The high beam radiated from the high beam filament 24 is reflected from the poly-ellipsoid reflecting mirror 20, and then, focuses on the second focus. After traveling through the lens 26, the high beam radiates nearly horizontally. FIG. 2b shows the horizontal cross sectional view of the optical design for the high beam. On the horizontal direction, the position of focusing is different from it on the vertical direction. After traveling through the lens 26, the high beam radiates with a wider angle. Referring to FIG. 2c, the optical design for a low beam is shown. Apart from a shade 28 located under the optical axis 22, the design is the same as FIG. 2a. With the existence of the shade 28, the light reflected upward from the mirror 20 is blocked. Therefore, the light radiated over the optical axis 22 (shown as a dash line) is very dim, and a contrast between upward and downward radiated lights is enhanced. The horizontal optical design of a low beam is similar to FIG. 2b. The description is omitted here. Using the design as shown in FIG. 2a, FIG. 2b and FIG. 2c, four PESs are needed for a car, and two PESs are needed for a motorcycle. Thus, a higher cost and a larger space are inevitable.
In U.S. Pat. No.4,985,816, an improved headlight design is provided. Low beam and high beam can radiate from the same PES by using a liquid crystal display (LCD) to obtain the required light distributing pattern. However, since the temperature of the headlight is so high, the normal LCD material is easily altered. While being radiated by high intensity of light, the contrast of the boundary for brightness is difficult to enhance.